1. Field of the Invention
The present invention relates to an image blur suppression device for a camera, and in particular, relates to a motion compensation device which compensates for motion, such as vibrations, in a camera which uses a simple sensor to detect both linear and rotational motion.
2. Description of the Related Art
Known motion compensation devices move a portion of an imaging optical system (hereinafter referred to as the compensation optical system) in a direction approximately perpendicular to the optical axis of the photographic lens, to prevent blurring of an image imparted by the movement of the camera during exposure. The compensation optical system is supported by a cantilever with elastic supporting material, having a high rigidity in the direction of the optical axis. Thus, when a force is added, in a direction approximately perpendicular to the optical axis, the compensation optical system moves within a plane that is approximately perpendicular to the optical axis of the imaging optical system (also referred to as the optical system).
Such a compensation optical system is typically driven by an electromagnetic actuator that utilizes a moving coil. In addition, when a magnetic circuit is formed between a magnet, which is polarized into two poles, and, for example, a yoke, and a signal is applied to a coil located within the magnetic lines of force, a magnetic force is generated in a direction perpendicular to the direction of the current flow. The direction of the magnetic lines of force are based on Fleming's left-hand rule. Therefore, a force in the direction of the X axis or the Y axis is generated when the coil is electrified.
Hand movement causes four types of blur during photo-taking: pitching blur, yawing blur, vertical blur and horizontal blur. Pitching and yawing blur is caused by the camera relatively moving about the optical axis, thereby making the image move on the film. Thus, for pitching or yawing blur, it is possible to stop the movement of the image on the film through cancellation, i.e. by driving a compensation optical system. FIG. 3 shows a block diagram showing an example of a motion compensation device in accordance with the prior art for use in an optical system of a camera. An angular velocity sensor 1 monitors the pitching and yawing of the camera. The angular velocity sensor 1 comprises, for example, a piezoelectric vibrator that detects Corioli's force. The output of the angular velocity sensor 1 is connected to an integrator 2.
The integrator 2 obtains the target drive location of the compensation optical system (not shown) by integrating the output of angular velocity sensor 1 over time and converting it to a blur angle of the camera for each pitch and yaw. The movement of the compensation optical system and the blur angle of the camera are related by Equation 1-1. EQU X=f.times..theta./.alpha. (Equation 1-1)
where:
X=movement of the compensation optical system (mm) PA1 f=Focal distance (mm) PA1 .theta.=Blur angle of the camera (rad) PA1 .alpha.=correction constant of the compensation optical system PA1 X=Movement of compensation optical system PA1 c=Viscosity coefficient PA1 A=Coil driving current PA1 k=Spring constant PA1 m=Mass of movable portion PA1 .alpha.=Driving power constant
A servo-circuit 3 compensates for the difference between the target location of the compensation optical system and the present location of the compensation optical system, by driving the compensation optical system of the camera based on the target location obtained by the integrator 2. The output of the servo-circuit 3 is connected to a signal driver 4. The signal driver 4 sends a signal corresponding to the input voltage, to a coil of an actuator 5. The actuator 5, for example, electromagnetically drives the compensation optical system, which is supported by an elastic supporting material using a cantilever and moving coil. Transmission function of such a compensation optical system against current is given by Equation 1-2. EQU X/A=.alpha./(ms2+cs+k) (Equation 1-2)
where:
The actuator 5 moves the compensation optical system, within a plane approximately perpendicular to the optical axis of the optical system, in accordance with the current flow in the coil. A location detecting sensor 6 optically monitors the movement of the compensation optical system, and provides feedback to the servo-circuit 3.
FIGS. 4 and 5 are diagrams showing the waveforms of blur experienced by a camera. FIG. 4 shows blur due to vertical hand movement, while FIG. 5 shows blur due to horizontal hand movement. As described above, there are four types of blur experienced by a camera: pitching blur, yawing blur, vertical blur and horizontal blur. Pitching blur and vertical blur cause vertical blurring of the image on the film. Yawing blur and horizontal blur cause horizontal blurring of the image on the film.
As shown in FIG. 4, the amount of vertical blur (dotted line) of the camera is approximately 20% of the total blur of the image on the film in the vertical direction (bold solid line) when it is converted to the movement distance of the image on the film. As shown in FIG. 5, the amount of horizontal blur (dotted line) of the camera is approximately 30% of the blur of the image on the film in the horizontal direction (bold solid line). In addition, the amount of vertical and horizontal blur, as a portion of the entire blur, increases as the focus length of the optical system decreases and the distance to the photo subject decreases.
In accordance with the prior art, image blur due to pitching and yawing can be corrected by monitoring with angular velocity sensor 1 and driving the compensation optical system. However, vertical and horizontal blur cannot be corrected as there is no sensor to sense vibrations causing the vertical and horizontal blur. In other words, for movement of the image in the vertical direction, even though the entire pitching component can be corrected by the compensation optical system, only approximately 80% of the total blur is eliminated. In addition, for movement of the image in the horizontal direction, even though the entire yawing component can be corrected by the compensation optical system, only approximately 70% of the total blur is eliminated. Furthermore, the percentage of the entire blur that can be corrected by detecting pitching and yawing, decreases as the focusing length of the optical system and the distance to the photo subject decreases.
In order to correct vertical and horizontal blur, an expensive special sensor that detects vertical and horizontal blur, for example a G sensor, is necessary. Other problems also arise that make such a motion compensation device even more expensive.